That's not good advice at all. Even with a slipper clutch you'll always want to rev-match because if you simply downshift while at high rpm, the chassis will unsettle. Yes, the slipper clutch should alleviate wheel-hop to a noticeable degree, but it will still cause the chassis to become unsettled, which is not the best thing while trail braking into a corner.
OEM slippers youll need to be more careful with, but when slipping each gear you are never fully disengaging, the rate of slip will remain very consistent even in between gears. If you have an aftermarket slipper you can grab 3 gears at once and just dump the clutch and forget about it.
Take a look at any professional racing or even club racing that goes on and you'll rarely, if ever, see someone simply downshift without rev-matching or relying on the autoblipper to do so. The fact that autoblippers exist negates what you're telling this person to do.
I dont know what races you are watching but no one is manually rev matching any more. Its slower then using a slipper or seamless. Even at the amateur level no one is rev matching if they have a slipper and do not have a auto blipper. You can hear it clear as day in the braking zones.
Slowly slipping the clutch is also not a good idea as you're then constantly loading and unloading the chassis. The goal is to have the chassis loaded in an even manner throughout the corner. Let's say you're braking from sixth to second gear and you're slowly slipping the clutch out for each gear. That's extremely inefficient and the bike will probably be seesawing as the chassis isn't being loaded at a constant rate from a lack of consistent engine braking. You're then trying to get the bike turned while doing all of that.
If you do it wrong sure, not if you do it correctly. Its very easy to keep it consistent.
It's better to build that braking and rev-matching skill now, as opposed to having someone try to break a bad habit once they've begun building up their pace. I see that stuff happen constantly. Learning how to blip and brake simultaneously is something that everyone needs to work on over their riding careers.
Been racing/track days for several years now and have never rev matched at the track once. I slip every downshift on an OEM slipper and the bike is always stable. Its technique, I can go from 6 - 2 and keep the bike dead stable. Theres no reason to take brain power away from braking to add rev matching. Even when I run my DRZ 400 which has no slipper im not rev matching. You get a feel for the clutch and slip it. Thats how guys are backing it in, its another control input.